The present invention relates to a distance measuring system with an electro-optical (light wave) distance measuring unit.
An electro-optical distance measuring unit provided in a distance measuring system is described, for instance, in the Japanese Patent Publication JP-A-2004-144681.
In FIG. 8, reference numeral 1 denotes an electro-optical distance measuring unit. The electro-optical distance measuring unit 1 comprises an optical system 2 and a distance measuring arithmetic unit 3.
The optical system 2 comprises a distance measuring light optical path 4 and an internal reference light optical path 5. On the distance measuring optical path 4, there are arranged a laser light source 6, a right angle reflection mirror 7, an objective lens 8, and a photodetection element 9. The laser light source 6 is a pulsed laser diode (PLD), for instance. From the laser light source 6, a distance measuring light, which is a pulsed laser beam, is emitted. The distance measuring light is deflected by the right angle reflection mirror 7, and the distance measuring light is projected from the electro-optical distance measuring unit 1 through the objective lens 8. The distance measuring light thus projected is reflected by a prism, which is an object to be measured 11. The reflected distance measuring light enters via the objective lens 8, and the reflected distance measuring light is then deflected by the right angle reflection mirror 7 and is received by the photodetection element 9.
On the distance measuring light optical path 4, a half-mirror 12 is arranged. By the half-mirror 12, a part of the distance measuring light is reflected as an internal reference light. The internal reference light is deflected by a reflection mirror 13, and the internal reference light optical path 5 is formed. Relay lenses 14 and 15 are provided on the internal reference optical path 5. After passing through the relay lens 15, the internal reference light 5 is deflected by a reflection mirror 16. The internal reference light 5 is then reflected by a half-mirror 17 arranged on the distance measuring optical path 4 on the distance measuring light optical path 4 and is received by the photodetection element 9.
An optical path switchover unit 18 is provided so as to stretch over the outgoing course of the distance measuring light optical path 4 and the internal reference light optical path 5. A light amount adjusting device 19 is provided so as to stretch over the returning course of the distance measuring light optical path 4 and the internal reference light optical path 5.
The optical path switchover unit 18 is comprises a rotating light shielding plate 21. When the light passes through one of the distance measuring light optical path 4 and the internal reference light optical path 5, the light shielding plate 21 shuts off the other of the optical paths. The rotating light shielding plate 21 is rotated by an actuator 22 such as a motor, etc. The light amount adjusting device 19 comprises a light amount attenuation filter 23. The light amount attenuation filter 23 is rotated by an actuator 24 such as a motor, etc., and the light amount is adjusted in such manner that the intensity of the distance measuring light entering the photodetection element 9 is to be equivalent to the intensity of the internal reference light.
The distance measuring light and the internal reference light received by the photodetection element 9 are sent to the distance measurement arithmetic unit 3 as photodetection signals.
As the optical paths are switched over by the optical path switchover unit 18, the distance measuring light and the internal reference light separated in time series enter alternately the photodetection element 9, and the photodetection signal of the distance measuring light and the photodetection signal of the internal reference light are alternately sent to the distance measurement arithmetic unit 3 from the photodetection element 9. At the distance measurement arithmetic unit 3, pulses of the distance measuring light and pulses of the internal reference light are compared with each other, and the deviation is calculated. Based on the deviation thus obtained, a distance to the object to be measured 11 is calculated.
In the electro-optical distance measuring unit 1 of a conventional type distance measuring system as described above, the distance measuring light and the internal reference light are mechanically switched over by the rotating light shielding plate 21. For instance, optical paths are switched over every one second, i.e. the distance measuring light is received for a second, and the internal reference light is received for the next second. When optical paths are mechanically switched over, there are such problems that there are limitations in the factors such as responsiveness of the motor. The switchover at high speed is very difficult to perform and influence of drift in a circuit unit of the distance measurement arithmetic unit 3 is not negligible, and the drift exerts influence on accuracy of distance measurement.
Further, during the switchover, there is also dead time when neither the distance measuring light nor the internal reference light is received, and the effective time for measurement is reduced. When the entire measurement time is reduced by switching over at high speed, there is such problem that an amount of measurement data acquired is decreased.